In the field of quantum devices, there have heretofore been made research and development on an electronic device using nanocrystalline silicons, such as an electron source (see, for example, Japanese Patent No. 2966842) or a light-emitting device (see, for example, Japanese Patent Laid-Open Publication No. H06-90019).
This type of conventional electron source comprises a lower electrode, a surface electrode (upper electrode) formed of a metal thin film and disposed in opposed relation to the lower electrode, and a strong electric field drift layer (hereinafter referred to as “drift layer”) disposed between the lower and surface electrodes. In the drift layer, electrons drift from the lower electrode toward the surface electrode in response to an electric field acting on the drift layer when a voltage is applied between the lower and surface electrodes in such a manner that the surface electrode has a higher potential than that of the lower electrode. In order to allow electrons to be emitted from the electron source, a collector electrode is disposed in a vacuum space to be opposed to the surface electrode. Then, a voltage is applied between the collector electrode and the surface electrode in such a manner that the collector electrode has a higher potential than that of the surface electrode while applying a voltage between the lower and surface electrodes in such a manner that the surface electrode has a higher potential than that of the lower electrode. In this way, electrons are injected from the lower electrode to the drift layer, and then emitted through the surface electrode after drifting in the drift layer.
The drift layer includes a number of nanocrystalline silicons. The surface-electrode is formed of a metal thin film (e.g. a gold thin film) having a thickness of about 10 nm. For example, the lower electrode of the conventional electron source is composed of a semiconductor substrate having a conductivity relatively close to that of conductor, and an ohmic electrode formed on the back surface of the semiconductor substrate. Alternatively, the lower electrode is composed of an insulative substrate (e.g. a glass substrate having an insulation performance, a ceramic substrate having an insulation performance etc.), and a conductive layer made of metal material and formed on the insulative substrate.
In the conventional light-emitting device, a pair of electrodes are provided, respectively, on both sides in the thickness direction of a luminescent layer including a number of nanocrystalline silicons. When a voltage is applied between the electrodes, the luminescent layer generates light, and the generated light is emitted through one of the electrode. This electrode is formed of a metal thin film having a thickness allowing the light to transmit therethrough. The metal thin film of the electron source or the light-emitting device is prepared through a sputtering method or the like.
Generally, it is desired that a metal thin film for use in a quantum device, such as an electron source or light-emitting device, has a reduced thickness as much as possible. However, if the metal thin film is thinned, it will have a poor coverage to a base (drift layer or luminescent layer), or will cause agglutination of its components due to surface tension and other factors, resulting in deteriorated durability of the electron source or light-emitting device. Further, in a process of vacuum-sealing the electron source, the metal thin film inevitably receives heat. Thus, the heat causes agglutination of the components of the metal thin film, and the resultingly lowered coverage to the base leads to deteriorated durability of the electron source or light-emitting device.
As one of solutions to these problems, it is conceivable to make the metal thin film of chromium having a higher coverage than that of gold. However, chromium is inherently susceptible to oxidation (or, poor in oxidation resistance). If the metal thin film is oxidized, the resultingly increased electrical resistance thereof will lead to deterioration in electron emission characteristics of the electron source or the luminescence characteristic of the light-emitting device, and to undesirably increased power consumption. In addition, chromium is strongly influenced by impurity gas (particularly, oxygen) or water residing in the vacuum space, which also leads to deteriorated durability of the electron source or light-emitting device.